1. Field of the Invention
This invention relates to a process for producing a high concentration aqueous solution of the salt of an alpha-sulfo fatty acid ester. More specifically it relates to a process for producing an aqueous solution containing a low concentration of the salt of an alpha-sulfo fatty acid and a high concentration of the salt of an alpha-sulfo fatty acid ester.
2. Description of the Prior Art
It is known in the art that an aqueous solution of the salt of an alpha-sulfo fatty acid ester can be produced by first sulfonating a fatty acid ester and, then, neutralizing the resultant sulfonated product with an aqueous caustic alkali solution, desirably after bleaching. It is also known in the art that the ester linkage tends to be cleaved during the sulfonation, bleaching and neutralization steps of this production process. The cleavage of the ester linkage necessarily causes the by-production of the salt of alpha-sulfo fatty acid. However, the salt of alpha-sulfo fatty acid is only slightly soluble in water and, also, has poor surface activity. Therefore, the inclusion of the by-product alpha-sulfo fatty acid salt in the aqueous solution of the salt of the alpha-sulfo fatty acid ester is not desired in the art.
Various attempts to prevent the cleavage of the ester linkage and to suppress the by-production of the undesired alpha-sulfo fatty acid salt have been proposed in the art. However, most of these proposals are concerned with improvements in the sulfonation or bleaching step and improvements in the neutralization step have scarcely been tried. The only improvement in the neutralization step is that the neutralization is carried out under relatively mild conditions to inhibit the cleavage (or hydrolysis) of the ester linkage during the neutralization step.
As is known, a high concentration is generally desired in the aqueous solution of a surface active agent. This is also true in the aqueous solution of the salt of an alpha-sulfo fatty acid ester. In the case where the concentration of the aqueous solution of the salt of an alpha-sulfo fatty acid ester is low, there is a disadvantage in the storage and the transportation thereof. The high concentration aqueous solution of the salt of an alpha-sulfo fatty acid ester can be theoretically obtained by concentrating from the low concentration aqueous solution or neutralizing the sulfonated products of a fatty acid ester with an high concentration alkali-solution. However, the former method is not recommended due to the fact that the content of the undesired alpha-sulfo fatty acid salt is increased by heating during concentration. On the other hand, the neutralization of the sulfonated fatty acid ester by using a high concentration aqueous caustic alkali solution has a disadvantage in that, since the viscosity of the solution is rapidly increased in the course of the neutralization step (even in the case where a relatively dilute aqueous caustic alkali solution is used), uniform agitation becomes difficult. As a result, an excessive alkaline region is locally formed in the aqueous solution to be neutralized and, therefore, the ester linkage is generally hydrolyzed. Thus, in the case where an aqueous caustic alkali solution having a high concentration is used, the undesirable hydrolysis of the ester linkage is accelerated. Accordingly, the use of a high concentration aqueous caustic alkali solution in the neutralization of the sulfonated fatty acid ester is not recommended.
As mentioned hereinabove, the production of an aqueous solution of the salt of an alpha-sulfo fatty acid ester containing a high concentration of the salt of an alpha-sulfo fatty acid ester (i.e. a desired surface active agent), but a low concentration of the salt of an alpha-sulfo fatty acid (i.e. an undesired impurity) is extremely difficult in the prior art.